1. Field of the Invention
The present invention relates to an electrocontact discharge dressing method for a grinding wheel provided with a grinding surface formed by bonding super hard abrasive grains with an electrically conductive bond such as a metal bond.
2. Description of the Prior art
A grinding wheel using super hard abrasive grains of diamond or cubic boron nitride has been widely employed for grinding hard materials such as advanced ceramics.
In order to improve the grinding ratio of this grinding wheel and prevent abrasive grains from releasing from the grinding wheel, and accordingly improve the economical efficiency, it is desirable to securely bond the abrasive grains with a metal bond or the like.
In order to improve the sharpness of the grinding wheel, it is required to be dressed before grinding so that the abrasive grains project from the metal bond by a predetermined amount. And for this purpose, conventionally, WA stick method using a white aluminum oxide (WA) stick has been generally used. However, this method has a problem that it is difficult to mechanically dress the metal bond super hard abrasive grain grinding wheel which uses a bond of high grade, and accordingly it takes a long dressing time so that the dressing efficiency is not good.
Another well known dressing method is an electric discharge dressing method of effecting an electric discharge between an electrode and the metal bond and melting the metal bond away from the grinding wheel due to the generated heat.
FIG. 8 illustrates one example of the device for the electric discharge dressing method. In FIG. 8, a base body 1 made of metal, which supports at one end surface a carbon electrode 12, is mounted on a table T of a surface grinding machine 7 through an insulating body 13. To an end of a wheel spindle 71 of the surface grinding machine 7 is secured a circular plate-shaped grinding wheel 5 having a grinding surface 51 in its outer peripheral surface. The grinding surface 51 is opposed to the electrode 12 through a predetermined gap g.
The base body 1 and the wheel spindle 71 are electrically connected to a power supply 8, respectively.
When a voltage is applied, an electric discharge is effected from the electrode 12 to a metal bond of the rotating grinding wheel 5 to melt the metal bond away from the grinding wheel 5 due to heat of the electric discharge. Thus, the grinding wheel 5 is dressed.
Electric discharge dressing as described above can be adapted to any grinding wheel of which the bond has electrical conductivity, regardless of the abrasive grains bonding force. However, this method has disadvantages of requiring a high discharge voltage(normally about 100 V), and being not free from the danger of receiving electric shock. Moreover, this method requires the alterations of the grinding machine. For example, a slip ring-brush mechanism 9 must be provided in the wheel spindle 71 in order to supply an electric current thereto, and insulating bodies 72 must be provided between the wheel spindle 71 and a main body of the grinding machine 7. And the production cost to the device provided with a pulse power supply is very expensive.
In addition to the above disadvantages, in the electric discharge dressing, an electric discharge tends to concentrate in the vicinity of the abrasive grains. This results in the bond 52 surrounding an abrasive grain 58 for example often excessively melting away to form a channel 54 around the abrasive grain 53 as shown in FIG. 9. The formation of channel 54 may often lower the abrasive grain retaining force.